Add test to dump hits from MultiSegment-MultiCollections example. Investigate issue: https://github.com/AIDASoft/DD4hep/issues/141 for details

Add DDRec/src/plugins to folders documented by doxygen

Add groups for plugins and some example
Add an entry in the main "Modules" part of the doxygen website so they are easily found.
And sub groups for different plugin types.
See changes in the plugins for example. The documentation in these changes is not complete.